Typically, base stations communicate with user equipments (UEs) in their corresponding operating areas or cells. A random access channel is used for carrying packets of data from a plurality of UEs on a contention basis. The random access channel is utilized in wireless communication systems such as the spread spectrum code division multiple access (CDMA) systems and the orthogonal frequency division multiplexing (OFDM) systems. In the exemplary embodiment, the CDMA systems are considered. However, the method can also be used in the OFDM systems. In CDMA systems, data signals are communicated between UEs and corresponding base stations over a shared spread spectrum.
Generally, in CDMA systems a random access channel (known as RACH) is used for carrying packets of traffic data from a plurality of UEs on a contention basis. In addition to handling call origination and registration messages, the RACH in Third Generation Partnership Project (3GPP) will carry traffic including Short Messaging Service (SMS) packets and possibly short data bursts in the absence of a dedicated traffic channel. The Third Generation Partnership (3GPP) is a collaboration agreement that was established in December 1998, from a co-operation between the European Telecommunications Standards Institute (ETSI), Association of Radio Industries and Businesses (ARIB) of Japan, CCS (China), Alliance for Telecommunications Industry Solutions (ATIS) of North America and TTA of South Korea. A goal of 3GPP was to make a globally applicable third generation (3G) mobile phone system specification within the scope of the International Telecommunication Union's (ITU) International Mobile Telecommunications-2000 (IMT -2000) project. 3GPP specifications are based on evolved GSM specifications, known as the Universal Mobile Telecommunications System (UMTS) system.
Because of such increase in traffic volume, accurately predicting performance of RACH and sizing communication resources appropriately has become important. The RACH, being an uplink transport channel, is generally characterized by a collision risk and by being transmitted using open loop power control. Each packet is distinguishable by a combination of time slots and codes. The transmission is time divided into repeating frames having time slots, such as fifteen time slots per frame. When a packet is transmitted over the RACH, it may last for multiple frames.
RACH throughput has commonly been assumed to be comparable to that of slotted ALOHA. ALOHA is a protocol for satellite and terrestrial radio transmissions. In pure ALOHA, a UE can transmit a message at any time but risks transmission collisions with other users' messages. A slotted ALOHA reduces the chance of collisions by dividing the channel into time slots and requiring that the UE transmits only at the beginning of a time slot.
Over the RACH, a random-access transmission is generally based on the slotted ALOHA approach with fast acquisition indication. A typical UE RACH access attempt is as follows. Prior to communicating over the RACH, a UE transmits an access signal to the base station or Node B to access the RACH. One type of access signal uses a preamble code in a preamble signal (hereafter referred to as the preamble). Generally, the UE repeats the preamble while incrementally increasing transmission power levels. The UE repeats transmission of the preamble until a response from the base station is received or until a maximum number of repetitions is reached.
Typically, when attempting to access the RACH, UEs randomly select the preamble sequence which corresponds to a determined resource in the base station. This randomness generally increases a collision probability between the UEs, and thus reduces the RACH throughput. Moreover, a period of time between access attempts is critical to a system's performance. If the period between access attempts is substantially long, the RACH will be underutilized. If the period is substantially short, an undesirably high number of UEs may repeatedly request access resulting in service interruptions.
A number of approaches, known in the available body of literature, aim to improve the RACH throughput. One such approach for controlling UE re-access attempts is to use a fixed backoff parameter. The UE typically reattempts access for a period of time based on the backoff parameter. The backoff parameter represents a deterministic wait period for an access reattempt. A problem with a fixed backoff parameter is a lack of adjustment in response to a cell loading. The cell loading refers to the number of UEs in established communication with a corresponding base station. Accordingly, during periods of light cell loading, the RACH may be underutilized and in periods of high loading service interruptions may result.
Another known approach is a rule based approach, in which the UE analyzes its prior access attempt statistics. Based on the access statistics, the UE determines a backoff parameter by applying predetermined rules. If the UE had many unsuccessful access attempts, the period between attempts is increased. Since the UE's prior access attempts may not represent current conditions, this rule based approach is not optimal.
Another known approach is a broadcast of a backoff parameter over a broadcast channel (BCH). The broadcasted backoff parameter is based on the RACH's loading, uplink interference level and other factors. The broadcasted backoff parameter is used to derive a backoff wait period at a targeted access time. However, due to delays in processing and transmitting the broadcasted backoff parameter, the broadcasted backoff parameter, which may not represent current conditions, is not optimal.
Accordingly, there is a need for addressing the problems noted above and others previously experienced.
Illustrative and exemplary embodiments of the invention are described in further detail below with reference to and in conjunction with the figures.